The present invention relates to aluminium alloys of the 6XXX series, to methods of processing such alloys and to a method for designing such alloys.
The 6XXX series aluminium alloys are aluminium based alloys that include magnesium (Mg) and silicon (Si), with the Mg and Si each generally being present in the range of 0.2 to 1.5% by weight.
The 6XXX series alloys are widely used in applications which require medium-high strength with good formability, weldability and extrudability. The applications include a wide range of architectual/structural/electrical applications. Typically, the 6XXX alloys are cast as billets and then extruded to form small round bars or other profiled shapes or forged (from extrusions or billets) into larger components.
Conventional theories of precipitation hardening in 6XXX series alloys state that hardening occurs via the precipitation and growth of Mg2Si in accordance with the following sequence:
i) Si atom clusters form during delay before ageing;
ii) GPI zones form during heat up to ageing temperature,
iii) GPII zones form-precipitation of xcex2xe2x80x3 Mg2Si;
iv) xcex2xe2x80x2 precipitate forms via transformation from xcex2xe2x80x3 and grows with the amount of xcex2xe2x80x2 depending upon the temperature and time; and
v) if overageing occurs, xcex2 Mg2Si precipitate forms.
As a consequence of the conventional theories that the ratio of Mg to Si in the precipitates that form in 6XXX alloys is approximately 2 (on an atomic weight basis), in order to produce alloys that are xe2x80x9cbalancedxe2x80x9d with respect to Mg and Si, the standard practice has been to calculate the relative amounts of Mg and Si to add to 6XXX alloys so that the alloys include atomic weight ratios of Mg:Si of 2:1.
In some instances, instead of forming balanced alloys, it is known to design 6XXX alloys to contain excess Si to increase the strength thereof. In this instance any Si that does not precipitate as Mg2Si or does not form intermetallics is free to form other phases, such as precipitates with other elements, which have an added strengthening effect. The level of excess Si is varied to produce the desired strengthening effectxe2x80x94with the limit of Si addition often being determined by factors such as the effect of Si addition on extrudability.
Other alloying element additions and heat treatment sequences of the 6XXX alloys are also predicated on the precipitation of Mg2Si. For example, maganese (Mn) can be added to alloys to produce a distribution of Mn which acts as heterogenous nucleation sites and increases the chance of forming xcex2xe2x80x2 Mg2Si rods. This significantly increases the flow stress for extrusion, but also increases the level of pinning of grain boundaries, and thus reduces or even prevents recrystallisation and course grain band formation.
There are a wide range of different options for processing cast billets of 6XXX alloys to manufacture final extruded or forged products.
By way of example, it is known to homogenise 6XXX series billets to dissolve the maximum possible amount of Mg and Si present as intermetallics at grain boundaries in the as-cast billets, producing a supersaturated solid solution which, upon cooling, produces uniform precipitation of intermetallics and Mg2Si. It also breaks up the cast structure and transforms AlFeSi intermetallics. This leads to greater uniformity of flow stress and final properties of the extrusions and allows the development of full mechanical properties. Typically, slow cooling rates, such as 100-200xc2x0 C./hour, are used.
Moreover, it is known to use induction heating to heat billets quickly to required temperatures before extrusion. Typically, gas heating is used to bring the billets to approximately 300xc2x0 C. and induction heating is used to complete heating billets to the extrusion temperatures. The rapid heat-up rate with induction heating does not allow sufficient time for xcex2xe2x80x2 Mg2Si precipitates to grow, and thus provides a fine dispersion for extrusion. Flow stresses are thus considerably reduced. Similarly, it is possible to maintain the same properties whilst using a substantially lower billet temperature, also allowing faster extrusion speeds to be used.
Furthermore, it is known to vary post-extrusion quenching rates depending on the alloy being extruded. A desirable feature of an alloy is that it has a low quench sensitivity, i.e. it can reach full properties with slow cooling. The benefits of this are that distortion can be minimised, properties are more uniform, and quenching equipment is not required.
There is a range of known practices for alloy selection, homogenisation, billet heating and quenching, and these are largely empirical optimisations within the boundaries of commonly used alloy systems. By way of example, practices, such as step cooling, slow cooling and fast cooling, are recommended after homogenisation.
Typical alloy specifications are provided in Table 1 for several alloys of the 6XXX series:
In the above table, unless ranges are stated, the amounts stated are maximum concentrations.
It has been discovered recently that age hardening of 6XXX series alloys does not occur by precipitation of Mg2Sixe2x80x94as has been previously accepted throughout the industryxe2x80x94but rather occurs via the precipitation of MgSi.
The discovered MgSi precipitation mechanism involves the nucleation and growth of xcex2xe2x80x2 MgSi precipitate with an Mg:Si ratio of 1 (atomic weight basis), and not 2 as previously believed, and comprises the following sequence:
i) formation of separate clusters of Mg and Si atoms;
ii) co-clustering of Mg and Si atoms, with the Mg:Si ratio increasing during low temperature ageing and eventually reaching 1;
iii) formation of small precipitates of unknown structure with a Mg:Si ratio close to 1;
iv) transformation of these precipitates to xcex2xe2x80x3 MgSi, with the ratio being 1; and
formation of xcex2xe2x80x2 and Bxe2x80x2 in the next stage of ageing, with the ratio of Mg and Si being 1.
One consequence of the above discovery is that current commercial 6XXX alloys that have been produced in accordance with conventional theories on the basis that they are balanced with respect to Mg and Si, i.e with Mg and Si precipitating as Mg2Si, in fact are not balanced.
Moreover, significantly, the applicant has found that better properties can be obtained with 6XXX alloys that are balanced with respect to Mg and Si, as this is now understood by the applicant. The properties of interest include, by way of example, extrudability, forgeability, conductivity, strength, and machinability.
According to the present invention there is provided a 6XXX series aluminum alloy containing Mg and Si which is characterised in that the Mg and Si that is available to form MgSi precipitates is present in amounts such that the ratio of Mg:Si, on an atomic weight basis, is between 0.8:1 and 1.2:1.
It is understood that for any given 6XXX series aluminium alloy the amount of Mg and Si that will be available to form Mg/Si precipitates will be less than the total amount of these elements added to the alloy composition. The reason for this is that there will always be a proportion (typically, relatively small) of the Mg and Si that remains in solution and a proportion of the Mg and Si that precipitate with other elements, such as iron (Fe) and copper (Cu), added to the alloys.
It is also understood herein that a 6XXX series aluminium alloy having Mg and Si that is available to form MgSi precipitates in amounts such that the ratio of MgSi is between 0.8:1 and 1.2:1 is a xe2x80x9cbalancedxe2x80x9d alloy with respect to Mg and Si and is in accordance with the discovered MgSi precipitation mechanism.
It is preferred that the ratio of Mg:Si be between 0.9:1 and 1.1:1.
It is preferred particularly that the ratio of Mg:Si be 1:1.
According to the present invention there is also provided a method of manufacturing an extruded product from a 6XXX series aluminium alloy which comprises the steps of:
i) casting a billet of a 6XXX series aluminum alloy containing Mg and Si as described above;
ii) extruding a final product shape from the billet; and
iii) heat treating the extruded product shape to precipitate MgSi.
The heat treatment step may be any suitable heat treatment.
According to the present invention there is also provided a method of manufacturing a forged product from a 6XXX series aluminum alloy which comprises the steps of:
i) casting a billet of a 6XXX series aluminium alloy containing Mg and Si as described above;
ii) forging a final product shape from the billet; and
iii) heat treating the alloy to precipitate MgSi.
The heat treatment step may be any suitable heat treatment.
The method described in the preceding paragraph may comprise extruding an intermediate product shape from the billet and thereafter forging the final product shape.
In order to investigate the present invention the applicant carried out a series of experiments and computer modelling on the 8 6XXX series aluminium alloys set out in Table 2 and 3 other 6XXX series aluminium alloys I, J and K with nominal Mg concentrations of 0.48 wt %, xcex5i concentrations of 0.8, 1.0 and 1.2 wt %, respectively, and concentrations of other elements of the order of the concentrations set out in Table 2.
Table 3 is a summary of the processing conditions for the alloys and the subsequent heat treatment.